As a servo system which is robust against external disturbances, there has been proposed a two-degree-of-freedom control system which can adjust its tracking property in dependence upon the presence of external disturbances (Transaction of SICE Vol. 28, No. 3, 343/350 (1992)).
According to such a conventional control system, a desired tracking property can be relatively easily obtained when the magnitude of the disturbances is not great enough to cause the control input to be saturated or reach its upper limit.
When such a control system is applied to a rear wheel steering system of a four wheel steering vehicle which steers the rear wheels with an actuator according to a steering command signal based on a predefined rear wheel steering model, since the external disturbances are affected by such factors as the vehicle speed, the friction of the steering mechanism, the road surface condition, and the tire properties in a highly complex manner, the actuator has to be provided with an extremely high output capacity for it to be capable of ensuring the ideal tracking property at all times. The need for such a high output capacity for the actuator is not economically acceptable in some cases, for instance when an electric motor is employed as the actuator.
The output capability of the actuator is naturally determined so that it may be sufficient in all the possible operating conditions. Thus, in a low speed range of the vehicle, for instance when the vehicle is stationary, the actuator is required to produce its maximum output. The steering load in a low speed range, for instance when the vehicle is stationary, may be as high as twice the normal load. In view of this fact, the inventors of this application have realized that the output capability of the actuator may be substantially reduced without any ill effect if the optimum tracking property is sought only in a high speed range where the steering load is relatively light, and that any favorable tracking capability or dynamic responsiveness is not important in a low speed range where the steering load is relatively heavy.